The invention relates to a rotor assembly for an electric motor comprising a rotor shaft and at least one permanent magnet arranged on the rotor shaft, as well as an inner-rotor electric motor in which such a rotor assembly is employed.
The invention relates to the area of brushless electric motors with permanent magnets and more particularly to d.c. motors configured as so-called inner-rotor electric motors. Inner-rotor electric motors comprise a rotor assembly including a rotor shaft and one or more permanent magnets arranged on the rotor shaft. The rotor shaft is inserted into a stator unit comprising a stator body and field windings.
The construction of such a motor is described, for example, in U.S. Pat. No. 5,970,600. The motor comprises a casing containing the stator, the rotor assembly as well as bearings to rotatably support the rotor assembly. The stator includes stator plates and windings and defines an inner space into which the rotor assembly can be inserted. As disclosed in the '600 patent, the bearings for the rotor assembly may be integrated into end caps of the motor casing yielding an overall compact construction.
A problem arises in the process of manufacturing of such an inner-rotor electric motor in that ferromagnetic particles are carried away from the rotor magnets when the rotor assembly is inserted into the interior space of the stator. These particles may reach the working gap defined essentially by the outer contour of the rotor magnet and the inner contour of the stator. It is in fact possible and common to protect the entire motor assembly against the penetration of foreign bodies and contaminants after assemblage by providing a casing as described in the '600 patent. However, during assemblage of different motor components, the inside of the motor and, more particularly, the working gap are not protected against the entry of such foreign bodies.
Moreover, inner-rotor electric motors according to the prior art require a complicated assembly process which is predominantly sequential, comprising a sequential pre-assemblage of the stator and of at least one bearing in the casing and a subsequent fitting of the rotor assembly into the stator and into the bearing. The concentricity of the parts that is required for the inner-rotor electric motor can only be attained by providing a lid-type flange containing the second bearing rotatably supporting the rotor shaft. This assemblage step is considerably complicated by the magnetic forces and moments issuing from the rotor magnet. These forces arise from the interaction of rotor magnets with the stack of stator plates and are directed radially and axially. As a result of this interaction, a contact-free concentric fitting of the rotor is either not possible at all, or is possible only with the deployment of considerable technical means.
German Patent No. 32 37 196 discloses a miniature synchronous motor which has a one-part cup-shaped casing surrounding the ironless field windings as a ferromagnetically active flux guide, and which further has a rotor arranged inside the field windings. The permanent magnets of this rotor are arranged on a sleeve of magnetically conducting material. The rotor is housed in an hermetically sealed casing containing the bearings. Torque output occurs via a permanent magnet coupling comprising a first element constituted by the permanent magnets of the rotor and a second element consisting of a permanent magnet array on a shaft outside the rotor case. By hermetically enclosing the rotor, it is guaranteed that foreign bodies cannot penetrate to the rotor and the bearings are permanently lubricated and protected against environmental impacts. Assemblage of the motor as a whole occurs by inserting the hermetically enclosed rotor into the cylindrical hollow space of a cup-shaped plastic shell open on one side and having embedded ironless field windings. The shell is held within a casing of ferromagnetic material that is also cup-shaped.
Relative to the '600 patent described above, the patent document DE 32 37 196 represents an improvement insofar as the motor assembly is simplified, while the shearing off of ferromagnetic particles from the rotor assembly occurring in the prior described assemblage is eliminated because field windings are ironless and are fully embedded into plastic material.
However, motors having ironless field windings, in accordance with the arrangement disclosed in DE 32 37 196, have a disadvantage of functioning with very low efficiency due to the large gap. Therefore, the disclosed construction is predominantly used in high-speed micromotors, typically for dental applications. They are entirely inappropriate for the delivery of larger torques such as required for instance in motor vehicle applications.
The inner-rotor motor of DE 32 37 196 has the further disadvantage in that a direct mechanical coupling of a load is not possible because of the hermetic enclosing of the rotor assembly. Therefore, the torque transfer from the rotor to a shaft can occur only indirectly, for instance by magnetic coupling, as described in the patent document.
It is a common practice in the field of pump motors to provide a gap tube or gap can for separating the stator and rotor so as to isolate the stand and the pump electronics from the side of the pump motor containing the conveyed medium. Such canned motors are described for instance in DE 38 18 582, EP 0 963 029, DE 199 07 555, and DE 44 34 448. All of these documents concern synchronous or asynchronous machines, wherein an essential feature of a disclosed canned motor is a gap can completely closed on one side.
U.S. Pat. No. 4,999,533 describes an electric motor with an enclosed rotor unit to be used, for example, in a blower. The rotor is enclosed by a sleeve having ribs engaging with the stator in order to support the stator relative to the rotor in a motor without frame.
The GB-A-1,330,674 also describes a rotor with a rotor assembly accommodated in a sleeve, the sleeve having stiffening ribs extending on its outside in a longitudinal direction.
In an older patent application of the same applicant, DE 100 34 302.3, a rotor assembly for an electric motor is described. The motor comprises a rotor shaft and at least one permanent magnet arranged on the rotor shaft. The rotor shaft with the permanent magnet are housed for support in a sleeve, the rotor shaft being freely rotatable within the sleeve.
The rotor assembly can be pre-assembled within the sleeve and is preferably designed such that the rotor shaft protrudes from the sleeve on one end of the sleeve. The sealing between the sleeve body and the rotor shaft is provided in bearings at the end faces of the sleeve rotatably supporting the rotor shaft. This arrangement does not result in hermetic sealing of the rotor assembly, however, with the bearings at the end faces of the sleeve being additionally protected by seals against the entry of contaminants, the sleeve is sufficiently protected against the penetration of foreign particles.
The rotor as a whole is inserted into the inside of the stator as a pre-assembled assembly, hence problems associated with a shearing off of ferromagnetic particles does not arise.
The rotor assembly that is enclosed by the plastic sleeve is self-centered in an axial direction by the magnetic forces acting between the rotor assembly and the stator when it is inserted into the stator. Thus, further precautions such as providing and adjusting stops for a correct positioning of the rotor in the stator need not be taken. Attention must merely be given to a sufficiently high free mobility of the rotor assembly within the stator, so that a stop, a closed end of the stator or the like does not prevent it from being magnetically centered inside the stator.
With such a rotor assembly, a distinct improvement over the prior art is achieved in that an inner-rotor electric motor can be assembled with little effort. It is also guaranteed that during assemblage and operation, no contaminants reach the inside of the motor or, more particularly, the working gap. However, these advantages result from utilizing additional parts, particularly the “lost” sleeve, which necessarily increases the manufacturing cost.